Spark-gap converter, induction-heating and melting assembly



- Oct. 23, 1956 M. ROWAN, JR., ETAL 2,768,226

SPARK-GAP CONVERTER, INDUCTION-HEATING AND MELTING AS SEMBLY Filed Aug. 27 1954 F|G.I I n WWII a Sheets-Sheet 1 IIIHHHHHIIIHI' IN V EN TORS HENRY M. ROWAN JR. PAUL E FOLEY ATTORNEYS Oct. 23, 1956 H. M. ROWAN, JR., ET AL 2,768,226

SPARK-GAP CONVERTER, INDUCTION-HEATING AND MELTING ASSEMBLY Filed Aug. 27, 1954 3 She'ets Shee-1 2 ATTORNEYS OctQ23, 1956 H M. ROWAN, JR., ET AL SPARK-GAP CONVERTER, INDUCTION-HEATING AND MELTING ASSEMBLY 3 sheets-s beet 5 Filed Aug. 27, 1954 FIG.7

TORS

QWAN JR. LEY

' INVEN ENR M.R PAUL A TTORNEYS United States Patent SPARK-GAP CONVERTER, INDUCTION-HEATING AND MELTING ASSEMBLY Henry M. Rowan, Jr., Trenton, N. J., and Paul F. Foley, Glenolden, Pa., assignors to Inductotherm Corporation, Glenolden, Pa., a corporation of Pennsylvania Application August 27, 1954, Serial No. 452,697 2 Claims. (CI. 13-46) The present invention relates generally to high-frequency induction-heating and melting devices and, more particularly, to improved apparatus constituted by a spark-gap converter and an induction furnace coupled thereto adapted to produce high-frequency power efiiciently and without excessive losses.

It is known to generate heating energy by means of a spark-gap converter yielding high-frequency oscillations which are applied to a furnace coil surrounding a metal work-piece to be heated or melted. The standard converter circuit arrangement comprises a high-reactance transformer, an electric-discharge gap device and a capacitor connected to the helix of the furnace, whereby the transformer charges the capacitor which thereafter discharges through the spark gap and the helix to set up damped oscillations.

In apparatus of the type heretofore known, the construction was such that substantial losses were encountered both in the spark-gap device and in the circuit conductors associated therewith. For example, conventional sparkgap devices customarily make use of a steel or cast-iron pot as a container for a mercury electrode. As a consequence of eddy current and magnetic losses induced in the pot, the efiiciency of the system was materially reduced. It was also standard practice to connect the converter to the furnace coil by means of relatively long, flexible cables extending along the floor to a separate melting table supporting the furnace. By reason of the cable reactances, further power losses were incurred.

Due to the losses inherent in conventional inductionheating and melting circuits, it became necessary, in order to attain higher power-levels, to operate the converter at relatively high voltages. Thus, in known industrial converters, the nominal operating voltages are usually in the order of 5,000 to 10,000 volts, with voltage peaks in excess of 20,000 to 30,000 volts, which voltage values bring about serious insulation problems and arcing difficulties. These drawbacks contribute to high furnace maintenance and high refractory costs. Moreover, the high operating voltages entail the use of capacitors and other components having heavy voltage ratings, further adding to the expense and bulk of the equipment.

In view of the foregoing, it is the principal object of the invention to provide an induction-heating and melting apparatus of high efficiency capable of operating in the larger power ranges at lower voltages than have hitherto been possible. It is to be understood that the apparatus in accordance with the invention is also applicable to sintering, alloying, brazing and various other heating operations.

More specifically, it is an object of the invention to provide a spark-gap converter assembly wherein the components thereof are so arranged and housed as to minimize losses due to lead length. A significant feature of the invention resides in the use of a melting table which also forms the top cover of the housing cubicle for the converter circuit. The furnace coil is detachably connected to the converter by means of short leads extending from sockets embedded in the table, the furnace coil prongs being received in said sockets. Thus, the resultant system is not only more efficient electrically, but hazards due to the presence of high-voltage cables along the floor are obviated.

Still another object of the invention is to provide a spark-gap device of sturdy and efiicient design wherein the pot for the mercury electrode is of non-magnetic and electrically-conductive construction, and wherein the cooling coils for the gap are embedded in the pot to afford a more effectively distributed cooling action.

It is also an object of the invention to provide a single chamber spark-gap device capable of increased currentcarrying capacity at lower voltages without increased power losses, and to provide effective and rapid powercontrol means without surging between chambers, as occurs in double-chamber gap devices.

Yet another object of the invention is to provide a highfrequency spark-gap converter installation of efficient and compact design and having small space requirements, which installation may be inexpensively manufactured and sold at reasonable cost.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in conjunction with the accompanying drawings wherein like components in the several views are represented by like reference numerals.

In the drawing:

Fig. 1 is a front elevational view of a converter installation in accordance with the invention, the interior being partially exposed to reveal the placement of parts.

Fig. 2 is a side elevational view of said installation.

Fig. 3 is a schematic circuit diagram of the spark-gap converter.

Fig. 4 is a schematic circuit diagram of a modification of the converter.

Fig. 5 is a longitudinal section of a spark-gap device in accordance with the invention.

Fig. 6 is a top plan view of said spark-gap device.

Fig. 7 is a longitudinal section taken along lines 7-7 in Fig. 6.

Fig. 8 is a section taken along lines 8-8 in Fig. 7.

Referring now to the drawing and more particularly to Figs. 13, the induction-heating and melting apparatus, in accordance with the invention, is housed in a relatively high rectangular cabinet 10 and a lower rectangular cubicle 11 attached thereto. In Fig. 1, the front panel of the cubicle 11 is removed to expose the parts therein. Cubicle 11 is provided with a top-cover plate 12 formed of Alberen stone or a material having similar heat-resistant properties. As is well known, Alberene stone is a natural quarried talcose rock which is homogeneous, dense and tough, and highly resistant to acid and alkali attack, even at high temperatures. Plate 12 serves as a melting table to support an induction furnace 13 which may be of any conventional design. Contained within cabinet 10 is a step-up power transformer 14, a cylindrical hydrogen tank 15, and a power-line switch 16. Mounted on the front panel 17 of the cabinet 10 is a power indicator in the form of a watt-meter 18. Housed within cubicle 11 are a pivotally-mounted spark-gap 19 and a bank of four condensers 20.

As shown in Fig. 3, the converter circuit comprises the main-line switch 16 which takes the form of a double-pole single-throw device connected to a line source 21 of single-phase alternating current. Transformer 14, which may be air-cooled or of oil-filled, water-cooled construction, is provided with a primary winding 22 connected through a current transformer 23 to switch 16. The output on current transformer 23 is coupled to the current terminals 24 of watt-meter 18, whereas the Voltage termi-' nals 25 thereof are connected across primary winding 22 of transformer 14. Thus, watt-meter 18 registers the voltage-current demand of the converter and afiords a power reading.

Secondary winding 26 of transformer 14 is connected in series with condenser 20 to the helix of furnace 13, the condensers being connected in parallel relation. Connected across secondary winding 26 are the electrodes of spark-gap device 19. Connection of the helix 27 to the converter is efiected by means of metal cups or contact sockets 28 and 29 embedded in the melting table and flush therewith. The helix 27 of furnace 13 is provided with terminal prongs 30 and 31 which are received within the mercury-filled sockets 28 and 29, respectively. Sockets 28 and 29 are connected by short bus-bars 69 to the busbars 70 leading to the components of the converter. It will be noted that spark-gap 19 is connected to bus bars 70 by means of flexible leads 71 to permit tilting of the device.

In operation, when the line-switch 16 is closed, voltage in the order, for example, of 4,000 volts is developed across the secondary winding 26, which voltage is impressed across condensers 20 via helix 27 to charge the condensers. When the voltage magnitude established at condensers 20 attains the breakdown value of spark-gap 19, it discharges therethrough, the discharge current fiowing through helix 27 to set up oscillations at the natural frequency of the circuit, until the power is dissipated in the furnace coil or as losses in the bus-bars. The oscillatory frequency depends on the circuit parameters and is preferably in a range of 20,000 to 80,000 cycles. The relative location of the furnace coil, the capacitors and the spark-gap and the inductive reactance of the bus-bars are of signal importance in minimizing losses and in realizing good efficiency at the higher currents resulting from the lower voltages. It will be appreciated that the use of the melting table as a cover for the cubicle and the placement of the condensers directly below the contact sockets for the furnace makes possible a considerable reduction in lead lengths.

Spark-gap device 19, which will be described in greater detail in connection with Figs. 8, is chiefly constituted by a solid electrode disposed above and spaced from a mercury-pool electrode, the electrode being contained in a hydrogen-filled chamber. The power output of the converter is controlled by adjusting the effective spacing between the electrodes, this being accomplished by tilting the spark-gap device and thereby changing the level of the mercury relative to the solid electrode.

Spark-gap device 19 is provided with trunnions 32 and 33 journaled within suitable bearings formed in a pair of spaced standards 34 and 35, mounted on stand-01f insulators 36 and 37, respectively. Tilting of the spark-gap device is effected by means of a handle 38 on the exterior of cabinet which is connected at one end by a pin 39 to a rocker arm 40 disposed within cabinet 10. The other end of arm 40 is pivotally connected to one end of a connecting rod 41 whose other end is pivotally connected to a lever 42 attached to trunnion 33. Pin 39 is rotatably supported within a friction bearing so as to maintain the tilt of spark-gap device 19 at any adjusted position thereof. It will be apparent that by shifting the angular position of handle 38, a similar tilt is brought about in the position of device 19. The tilt is preferably limited to an arc of approximately 45 degrees to prevent physical contact between the electrodes.

The furnace l3 basically acts as an air-core transformer whose primary is constituted by the helix coil 27 and whose secondary is the metallic body to be heated or melted. Helix 27 is formed of copper tubing, insulated between turns, through which water is conveyed for cooling the copper, pipe 43 being the water inlet to the helix. Disposed within helix 27 is a crucible 44, shown in dotted lines in Fig. 2, the crucible being fabricated of a substance which is suitable for the metal charge therein to be heated. When high-frequency currents are applied to the prongs 30 and 31 of the helix, all the space within the helix is subjected to an electromagnetic field which acts to induce currents in the metal charge contained in the crucible. These induced currents in the charge effect rapid heating thereof up to and above the melting point of the charge.

The increased eificiency attending the above-described arrangement has made it possible in practice to operate a converter at voltage levels below 5,000 volts and yet to achieve power-ratings in excess of 8,000 watts. It is to be understood, however, that the invention is by no means limited to operation below 5,000 volts, and the invention may be used to advantage at higher-voltage levels.

In the circuit of Fig. 3, the ratings of the condensers 20 must be such as to support the full voltage of the transformer. Thus, assuming a transformer voltage output of approximately 4,000 volts, condenser ratings of at least 4,400 volts are desirable. Alternatively, as shown in Fig. 4, the condensers 20 may be connected in pairs on either side of helix 27, so that the condensers are in series relative to the secondary 26. Thus, the power requirements of the condensers are cut in half and 2,200-volt condensers are adequate in this circuit arrangement.

Referring now to Figs. 5 to 8, there is shown a sparkgap device in accordance with the invention, comprising a solid electrode 45 disposed within a pot or container 46 in spaced relation to a liquid electrode 47 formed by a pool of mercury. Electric discharge takes place between the solid and the liquid electrode. Container 46 is constituted by a hollow cylinder closed at either end and formed of a non-magnetic metal, such as cast aluminum, which is also highly conductive both thermally and electrically.

Solid electrode 45 is shaped from a solid metallic block, preferably of copper, the lower end of the electrode being tipped with a face plate 48, preferably of tungsten. The body of the block is tapered inwardly in the direction of the upper end thereof, the taper terminating in a flange 45a. The taper functions to minimize the tendency of the arc to climb upwardly on the electrode. Solid electrode 45 is mounted at the lower end of a vertically-supported cylindrical porcelain or ceramic insulator 49 projecting into the gap chamber through a central opening 46a in the top wall of container 46. Surrounding insulator 49 at an intermediate point thereon is a non-magnetic metal gasket 50 having a base flange 50a encircling opening 46a and secured to the top wall of container 46 by suitable screws 51. The portion of insulator 49 extending above gasket 50 is corrugated to increase the insulating surface thereof, the remaining portion being smooth.

Extending coaxially through a longitudinal passage in insulator 49 is a pipe 52, preferably of copper, the lower end of the pipe being received Within a central recess 45b within electrode 45 and being brazed or otherwise secured to the wall of the recess. A non-metallic washer 53, such as one formed of asbestos, is interposed between the upper surface of flange 45a and the lower end of insulator 49. Concentrically disposed within pipe 52 is a metal tube 54, the lower end of the tube opening into recess 45b. A hose nipple 55 is connected to the upper end of tube 54 for coupling a water inlet, and a hose nipple 56 is connected to the pipe 52 for coupling to a water outlet. Thus, the solid electrode 45 is cooled by water flowing through the tube 54 into recess 45b, the water then passing through the space between tube 55 and pipe 52 to the outlet. The space between the outer wall of pipe 52, and the inner wall of insulator 49 is filled with a suitable sealing compound 57.

Received within a threaded opening in the top wall of container 46 is a sleeve 58, the sleeve extending above the top wall and being closed at the upper end by a replaceable blow-out window 59, preferably of mica. Blow-out window 59'is secured to sleeve 58 by means of a removable cap 60. Inasmuch as the gap chamber is filled with hydrogen, the blow-out gap acts as a relief valve to vent the interior of the chamber in the event of a gas explosion to protect the components from damage or destruction.

An atmosphere of hydrogen is maintained in the chamber, and for this purpose there is provided an inlet nipple 61 and an outlet nipple 62, both extending from the top wall of container 46. The manner of maintaining and regulating the hydrogen atmosphere forms no part of the present invention; hence, save for the gas tank shown in Fig. 1, the elements of the gas system are not disclosed herein. The hydrogen-flow-control system may employ mercury bottles similar to that shown in U. S. Patent No. 2,069,495, issued February 2, 1937.

Laterally extending from opposing sides of container 46 are trunnions 63 and 64, to permit tilting of the chamber, in the manner disclosed in connection with Figs. 1 and 2. Embedded in the cylindrical wall of container 46 is a helical cooling coil 65, preferably formed of stainless steel tubing. A water-inlet pipe 66 projecting from the side of the container 46 adjacent the top wall thereof is coupled to the upper extremity of the helical coil 65. The lower extremity of the coil 65 is integral with one end of a sinuously-shaped cooling coil 67 embedded in the bottom wall of the container, as shown separately in Fig. 7, the other end of coil 66 terminating in an outlet pipe 68. Thus, to remove radiant heat from the arc, water circulates not only along the cylindrical shell of container 46 but also within the bottom wall thereof to provide a highly effective and uniformly-distributed cooling action in the gap chamber.

Electrical connection to the liquid electrode 47 is effected through the conductive container 46, the lead thereto being preferably attached to the bearing for one of the trunnions. Electrical connection to the solid electrode 45 is effected by attaching a lead to the metallic, nonmagnetic nut, threadably received on pipe 52. The tilting of the device atfords rapid control between no-load to full-load conditions.

It is important to note that the use of a non-magnetic, conductive spark-gap device reduces heat losses to a substantial degree and at the same time provides a simple and effective means for connection to the liquid electrode. In lieu of a cooling coil embedded within container 46, a cooling duct may be integrally cast therein to preclude the use of any magnetic components in the spark-gap device.

Thus, there has been disclosed what is at present considered to be a preferred embodiment of the invention. It will be appreciated that the use of the cubicle cover as the melting table for the furnace, and the resultant reduetion in lead lengths in the converter circuit makes possible a highly compact and electrically efficient assembly. The efiiciency of the converter is further enhanced by the use of a non-magnetic, conductive spark-gap device. It will be apparent that many changes and modifications may be made in the assembly without departing from the essential spirit of the invention, and it is intended in the annexed claims to cover all such changes as fall within the true scope of the invention.

6 We claim:

1. A spark-gap converter installation comprising a high-frequency generator including a spar -gap device, a condenser, and a step-up transformer, a housing for said generator including a relatively high rectangular cabinet enclosing said transformer, and a relatively low rectangular cubicle attached to said cabinet and enclosing said spark-gap device and said condenser, said cubicle including a top cover forming a melting table, a furnace supported on said table and provided with a coil and external prongs therefor, a pair of mercury-filled sockets embedded in said table to receive said prongs, relatively short conductors connecting said sockets to said spark-gap device and said condenser to define an oscillatory circuit in which said condenser is dischargeable through said coil and said device, said spark-gap device being constituted by a mercury electrode and a solid electrode within a container, and means including a lever mounted externally on said cabinet for shifting said container to adjust the gap length therein.

2. A spark-gap converter installation comprising a high-frequency generator including a hydrogen-filled spark-gap device, a condenser and a step-up transformer, a housing for said generator including a relatively high rectangular cabinet enclosing said transformer and a hydrogen source for said device, and a relatively low rectangular cubicle attached to said cabinet and enclosing said spark-gap device and said condenser, said cubicle having a heat-resistant cover forming a melting table, a furnace supported on said table and provided with a crucible, a coil surrounding said crucible and having external terminal prongs, a pair of mercury-filled sockets embedded in said table to receive said prongs, relatively short conductors connecting said sockets to said sparkgap device and said condensers to define an oscillatory circuit in which said condenser is dischargeable through said coil and said device, said device including a solid electrode spaced from a mercury electrode within a nonmagnetic, conductive container having b1ow-out means and hydrogen flow means, said container having a helical cooling tube embedded in the walls thereof, and means including a lever mounted externally on said cabinet to shift said container to adjust the gap length therein thereby to control the power of said converter.

References Cited in the file of this patent UNITED STATES PATENTS 1,131,434 Snee et a1. Mar. 9, 1915 1,182,291 Meikle May 9, 1916 1,286,395 Northrup Dec. 3, 1918 1,451,271 Rentschler Apr. 10, 1923 1,572,873 Allcutt Feb. 16, 1926 1,795,926 Brace Mar. 10, 1931 1,801,538 Briscoe Apr. 21, 1931 2,069,495 Kennedy Feb. 2, 1937 

